The present invention relates to a method and apparatus for controlling a direct current (DC) electric motor and, more particularly, to a method and apparatus for controlling a DC motor to operate along preferred portions of two or more operating curves for the DC motor to thereby prevent stalling of the DC motor due to operation along an operating curve which is less efficient than one or more other operating curves for the DC motor. While the present invention is generally applicable to DC motor control, it will be described with reference to a DC electric motor for use in a windshield wiper system of a motor vehicle.
In motor vehicle windshield wiper systems, differing speeds for windshield wipers are often achieved by inducing inefficiencies into the wiper DC motors. One example of induced inefficiency is displacement of armature brushes from maximum efficiency with higher speeds resulting from greater displacements. While higher wiper speeds are required to clear the windshield when substantial moisture is being continuously accumulated on the windshield, oftentimes an operator of a motor vehicle will not reduce the speed of wiper operation as the moisture accumulation is reduced.
Also, as the moisture on the windshield is reduced, the load on a DC electric motor driving the windshield wipers increases dramatically for example when going from a wet wiping load to a damp or even dry wiping load. In a wet wiping load, the water on the windshield serves to lubricate the wiping of the wiper blades over the windshield. For damp or dry conditions, the lubrication of the water is reduced or gone altogether.
When considered together, it is apparent that a preoccupied motor vehicle operator who fails to reduce the wiper speed as the windshield progresses from a wet condition to a damp or dry condition is placing substantially greater loads on the windshield wiper motor while operating the motor along an operating curve which may be substantially less efficient than one or more available operating curves. Continued operation of the windshield wiper motor under these conditions can lead to motor stalling, overheating and burnout if not otherwise protected.
A number of automatic or self-acting arrangements have been proposed in the prior art to overcome these problems and protect windshield wiper motors. Such arrangements include: temperature sensor interrupters, which periodically stop a windshield wiper motor in the case of overheating; a circuit which de-energizes a motor when overloaded or stalled; a circuit which switches from continuous speed to intermittent operation upon overload and adjusts the period of intermittency to prevent overheating or destruction of the motor; and, a circuit which senses moisture on the windshield and changes the speed of a windshield wiper motor dependent upon the sensed moisture.
While these prior art arrangements assist in preventing motor overheating and burnout, a need remains for an arrangement which improves motor operation during overload conditions by fully utilizing the operating efficiencies of the motor throughout its normal operating range. Preferably such an arrangement would be automatic or self-acting to control motor operation such that the most efficient available operation is utilized in accordance with commanded operations and currently existing motor conditions such that the motor is operated along preferred portions of two or more operating curves for the motor. Further, switching between or among two or more operating curves would be performed to be substantially unobservable or transparent to the user of the motor.